The present invention relates to a method of manufacturing an optical disc master plate, particularly to a method of manufacturing an optical disc master plate employing a SIL (Solid Immersion Lens).
FIG. 7 is an explanatory view illustrating some important steps involved in a conventional method for manufacturing an optical disc master plate with the use of an SIL.
Referring to FIG. 7a, a glass substrate 1 having a predetermined configuration is first coated with an organic Primer such as HMDS (Hexamethyl Di-silazane), and is then spin-coated with a photoresist material dissolved in a solvent, so as to form a predetermined photoresist layer 2 tightly adhered to the surface of the glass substrate 1.
Subsequently, the solvent is removed by virtue of evaporation. Then, a clean oven is used to pre-bake the glass substrate 1 together with the photoresist layer 2, thereby forming a stabilized photoresist layer 2 on the glass substrate 1.
FIG. 8 illustrates a function of an optical system within a recording head. As shown in FIG. 8, a recording beam L modulated in accordance with the information to be recorded is directed so as to pass through an objective lens 5. Thus, the recording beam L is converged, and is then collected on substantially the center of the emitting surface of the SIL 4. If the SIL 4 is a super spherical lens, an equivalent NA (Numerical Aperture) of an optical system using the SIL 4 may be indicated by the following equation. EQU NA=n.sup.2.times.NA
Here, n is index of refraction of SIL 4. Therefore, a diameter L of the beam spot of the recording beam may be represented in the following equation, where .lambda. is wavelength of the recording beam. EQU L=.lambda./(n.sup.2.times.NA)
Subsequently, in an exposure treatment shown in FIG. 7b, a small interval (having a width which is 1/10-1/4 of the wavelength .lambda. of the recording beam) is formed between the SIL 4 and the photoresist layer 2, by virtue of a flying head 3 and an air flow induced by the rotation of the master disc plate. When a light beam exiting from the SIL 4 has a diameter D1 (FIG. 9) and the above small interval is less than 1/4 of the wavelength .lambda. of the recording beam, a beam spot on the photoresist layer 2 will also has a diameter of D1.
In this way, recording beam L converged by an objective lens 5 is further converged by SIL 4, and is caused to irradiate the photoresist layer 2 on the glass substrate 1, so that information may be recorded by cutting pits spirally thereon.
After that, referring to FIG. 7c, a developing process is conducted, thereby obtaining an optical disc master plate on which a plurality of pits are formed in a spiral array.
However, when employing a pit cutting device using the SIL 4, since the SIL 4 serves to perform a near field recording (near the photoresist layer 2), it is necessary that the interval between the light exiting surface of the SIL 4 and the photoresist layer 2 of the glass substrate 1 be constantly kept at a width which is 1/4 of the wavelength .lambda. of the recording beam.
In this manner, once the photoresist layer 2 of the glass substrate 1 bears a static electricity, a force caused due to such static electricity will act between the SIL 4 and the photoresist layer 2. As a result, it is difficult to stably keep the above interval between the light exiting surface of the SIL 4 and the photoresist layer 2 of the glass substrate 1. Moreover, dust or small rubbish will adhere to the photoresist layer 2, causing a collision between the SIL 4 and the dust or small rubbish attached on the photoresist layer 2, making it difficult or even impossible to perform a predetermined information recording.